Abstract 5677: MB101, a novel PD-L1 x CD3 targeted bispecific antibody, exhibits potent tumor-killing efficacy with minimal off-tumor toxicity

Bispecific antibody (bsAb) which targets tumor-associated antigens and simultaneously induces CD3-mediated T cell activation, is one of the promising therapeutic approaches for cancer treatment. However, despite numerous CD3-based bsAb have been developed, the clinical application faces pronounced hurdles, such as increased on-target off-tumor toxicity and cytokine release syndrome (CRS) by using high-affinity anti-CD3 moieties. Accordingly, there are still needs for the development of CD3-based bsAb with more favorable properties, which are tumor-specific targeting and potent tumor killing activity without excessive cytokine release. Thus, we developed MB101, a novel PD-L1 x CD3 bsAb with low affinity to CD3, which is uncoupled potent tumor cytotoxicity from severe cytokine release. MB101 showed a strong binding affinity to PD-L1 (KD = 0.4 nM) but a weak binding affinity to CD3 (KD = 1.0 μM), indicating preferentially targeting to PD-L1+ tumor cells. MB101 showed an effective tumor cell killing activity with higher Emax compared to avelumab mono- or combination treatment with CD3 mAb in PD-L1+ MDA-MB-231 cells. T cell-mediated cytotoxicity stimulated by MB101 in co-culture with target cells was accompanied by lower secretion of pro-inflammatory cytokines than CD3 mAb and bsAb comparator with high-affinity anti-CD3 arm. Additionally, in the absence of target cells, MB101 treatment to PBMC induced a very low level of pro-inflammatory cytokines, suggesting the possibility of reduced CRS risk. Furthermore, we proved that T cell-mediated cytotoxicity by MB101 was increased in a PD-L1 expression-dependent manner of tumor cells, but not normal cells, indicating minimal on-target off-tumor toxicity. Consistently with in vitro results, MB101 effectively regressed tumor volume (TGI=83%) without induction of excessive cytokine release in human CD3 knock-in mouse model. In addition, MB101 showed superior tumor growth inhibition (TGI=70%) than that of avelumab (TGI=44%) in humanized mouse xenograft model. In summary, MB101 selectively triggers robust PD-L1-dependent tumor cytotoxicity while inducing very low cytokine release, suggesting that MB101 might be a promising therapeutic option as CD3-based bsAb for cancer treatment. Citation Format: Suna Kim, Hyojoo Bang, Yongjun Jung, Sunjung Cho, Youngsam Kim, Sang-hyun Park, Seok Chan Kang, Youngjin Park, Sungyoub Jung. MB101, a novel PD-L1 x CD3 targeted bispecific antibody, exhibits potent tumor-killing efficacy with mi